Combustion exhaust gas treating apparatus for incinerators for refuse such as garbage are adapted to discharge combustion exhaust gas from a combustion unit 101 through a chimney 106 by means of an induced draft blower 105 as shown in FIG. 15. A flue 102 for conducting the exhaust gas from the combustion unit 101 is provided as arranged one after another with a unit 103 for adjusting the temperature of the exhaust gas, a bag filter 120, a unit 122 for heating the exhaust gas and a catalytic reactor 121. The temperature adjusting unit 103 cools the exhaust gas once, followed by removal of soot and dust from the gas by the bag filter 120 and then by reheating of the gas by the heating unit 122. Harmful substances such as nitrogen oxides and dioxin are thereafter removed from the exhaust gas by the catalytic reactor 121.
The combustion exhaust gas is cooled first because the upper limit temperature at the inlet of the bag filter is about 150 to 200.degree. C., and the gas is reheated because the temperature permitting the catalyst to retain its activity is about 210 to 450.degree. C.
The exhaust gas treating apparatus thus cools and reheats the exhaust gas and therefore has the problem that the exhaust gas treatment entails an undesirable heat loss. However, if the catalytic reactor is disposed to the front of the bag filter, soot and dust will adhere to the surface of the catalyst within the reactor to rapidly impair the properties of the catalyst, so that it is impossible to change the order of arrangement of the bag filter and the reactor.
Further since the bag filter has the pores of its bag clogged with adhering soot and dust, the filter needs to be periodically treated by reverse-pressure cleaning. The catalyst in the reactor must have its surface regenerated. The provision of the bag filter for dust removal and the catalytic reactor for making the exhaust gas harmless renders the equipment large-sized and more costly. Moreover, a great heat loss will result if the exhaust gas is reheated to an excessively high temperature, whereas lower reheating temperatures make it necessary to increase the area of contact between the catalyst and the exhaust gas in order to enable the reactor to function satisfactorily. This entails the problem of making the reactor greater in size.
To overcome the above problems, it has been proposed to use a bag of inorganic material such as a heat-resistant ceramic fiber as the bag filter, but it is then difficult to prevent the alterations in the pore size of the ceramic filter due to reverse-pressure cleaning since the filter is made also of a fabric, hence the problem of low stability in removing fine dust.
An object of the present invention is to eliminate the foregoing drawbacks and to provide means for reliably removing fine dust from the combustion exhaust gas, the means being also adapted, when required, to render harmless the harmful substances contained in the exhaust gas.